P-cells in the lateral geniculate nucleus (LGN) with foveal or para-foveal receptive fields (RFs) typically have chromatically-opponent centre-surround organisation. How this opponency arises is a matter of considerable debate. It might arise from a wiring whereby the cone inputs are selected in a principled fashion to allow for opponency. Alternatively, since the RF centre at these eccentricities is thought to receive input from a single cone, and the surround from very few, the chromatic opponency might arise simply from random wiring of the cone outputs to retinal ganglion cells. Therefore, the random wiring hypothesis predicts reduced chromatic opponency in the periphery versus central foveal vision. Electrophysiological studies have provided conflicting evidence for that hypothesis (e.g. see Dacey, 1996; 2000 versus Martin et al, 2001; Solomon et al, 2005).

The theories might also be distinguished psychophysically. We quantified chromatic opponency in central and peripheral regions by measuring the effect of retinal after-images for chromatic stimuli equated in detection thresholds. Subjects adapted a patch of retina at either 7.5° or 30° from fixation on the vertical meridian. Two probes were then presented, one falling in the same location as the preceding adapter and the other falling in the corresponding location in the other hemifield. We measured the point of subjective equality (PSE) between the two fields for a full range of chromatic probes. A reduction in opponency at eccentric locations would predict that the after-image would be weaker or less tightly tuned than in the more central location. We found no evidence to suggest a weakened chromatic opponency across the visual field, in agreement with Solomon et al (2005).